The present invention relates to a liquid distributor for an evaporator, which liquid distributor is particularly well suited to a film evaporator meant for liquid suspensions containing solid particles and incorporates a channel array located at the top of the liquid-evaporating heat transfer surface for distributing the liquid along the whole width of the surface, said channel array comprising a transversal supply channel and a number of distribution channels branching off downwardly therefrom and leading to the heat transfer surface.
In an evaporator where liquid flows as a thin film on the heat transfer surface, the whole surface must be covered with liquid in order to achieve an efficient heat transfer. Moreover, while evaporating a liquid suspension containing solid particles, growing solid contents at the borders of dry areas and areas covered with liquid would lead to a situation where the heat transfer surface would get dirty very rapidly. Thus an even distribution of the liquid to be evaporated is an absolutely necessary condition for an efficient operation of the evaporator.
The state where the heat transfer surface is covered with water can be maintained and improved by increasing the amount of liquid to be fed on the surface. This, however, also increases both the energy consumption in the pumping and the thickness of the flowing liquid film, which then weakens the heat transfer coefficient. Therefore it has been customary in liquid distributors of evaporators to use fairly narrow liquid channels, which are then easily choked while evaporating solids-containing liquid suspensions.
From the Finnish publication print 86,961 there is known a heat exchanger meant for evaporating liquids and comprising a number of adjacent bags made of a plastic film, and the film surfaces serve as the heat transfer surfaces. The liquid to be evaporated is fed onto the outer surfaces of the bags through a channel system provided in a honeycomb structure located at the top end of the evaporator, the adjacent channels extending over the whole width of the bags. The employed heating medium is steam conducted into the bags via the same honeycomb structure, said steam being created in the evaporation process and compressed in a compressor.
The apparatus described in the FI publication 86,961 is meant for distilling sea water into drinking water. Another application mentioned in said publication is the concentration of solutions and suspensions, such as bleaching effluents. However, in this case the apparatus has the drawback that the fiber material and other solid particles contained in the suspension tend to block the narrow, diagonal liquid channels provided in the honeycomb structure.
The FI patent application No. 944,471, identified for priority in International Publication No. WO96/09872 published on Apr. 4, 1996, which is related to the present invention in a way stipulated under .sctn. 2, paragraph 2 of the Finnish Patent Law, describes a liquid distributor for an evaporator, which is particularly designed for the treatment of fiber-containing effluents of the wood-processing industry, such as spent lye or bleaching effluents from chemical pulp digestion, or for the treatment of food industry sewage forming carbonate, oxalate and other deposits. This liquid distributor particularly aims at avoiding the problem of blocked channels caused by the solid particles contained in the suspension under treatment. Said liquid distributor comprises a diagonally downwards inclined supply channel and a number of distribution channels branching off therefrom and leading to the heat transfer surface, where the respective flow division takes place on the rounded division surface located on the downstream side with respect to the supply flow at the channel junction. By means of the described arrangement, on the liquid flow path there are eliminated sharp edges to which the solid particle accumulations could stick to.
In the liquid distributor specified in the FI application No. 944,471, the width of each vertical distribution channel is constant for the whole length of the channel after said rounded division surface. Practice has shown that this type of channels have the drawback that if the channel is completely filled with liquid, there is created a strong suction owing to siphonage, and this suction prevents the desired division of the flow on the division surface, while the distribution channel absorbs an excessively high proportion of the liquid flow. If such a siphonage flow is created in one of the distribution channels first branching off of the supply channel, the result is that the peripheral area of the heat transfer surface obtains an excessive amount of liquid to be evaporated, whereas the amount of water distributed on the middle areas is slight, or the middle area may even remain partly dry. Hence the aim for an even liquid distribution is not achieved.